The present invention relates to a method and a device in accordance with the generic terms of the independent claims. Thus the present invention is concerned with the detection of three-dimensional objects, in particular with the determination of the spatial depth of an expanded three-dimensional object.
It is frequently desirable to scan a given object three-dimensionally. For example, this is the case when a complicated workpiece should be inspected to determine whether it has been created accurate to dimension or when a given object is to be measured precisely.
As a rule it is possible to achieve a two-dimensional image of a three-dimensional object by means of projecting an object image to a sensor surface or the like. On the other hand, measurement into a third dimension, namely the measurement of a spatial depth, as a rule causes greater problems. For example, it has been proposed to send short light impulses out as with a sonic altimeter and to measure the time until reception of the backscattered or reflected impulses. However, measurement with light is very difficult here due to the extremely low run times, in addition room depth information can only be gained from spatially expanded objects with great expenditure.
There are also interferometric methods that have been known for a long time, in which a light beam is split up into a reference light beam and an object light beam. The object light beam is irradiated onto an object and received back from it. The reference and object light beams are then superimposed at a light receiver and then, provided there is sufficient coherence of both beams, the distance of the object is inferred from the phase location. This method allows high-precision measurements; however the depth measurement causes difficulties with expanded objects at different places.
In addition, performing distance measurements using frequency-shifted feedback lasers (FSF laser) is also well known. An example of this can be found in the essay by K. NAKAMURA, T. MIYAHARA M. YOSHIDA, T. HARA and H. ITO “A new technique of optical ranging by a frequency-shifted feedback laser” IEEEE Photonics Technology Letters, volume 10, 1998, pages 1772 pp. The principle of FSF lasers and the emission obtained from them is described in detail in the essay “Observation of a highly phase-correlated chirped frequency comb output from a frequency-shifted feedback laser” by K. NAKAMURA, T. MIYAHARA and H. ITO, Applied Physics Letters, Volume 72, No. 21, pages 2631 pp. as well as in the essay “Spectral Characteristics of an All Solid-State Frequency-Shifted Feedback Laser” by K. NAKAMURA, F. ABE, K. KASAHARA, T. HARA, M. SATO and H. ITO in IEEE-JOURNAL OF QUANTUM ELECTRONICS, Volume 33, pages 103 pp.
The authors of the documents above-named that have been included in their entirety for publication purposes proposed using an FSF laser, which has an acousto-optical modulator in its optical resonator, which is operated at about 80 MHz. The beam of the FSF laser is split at a beam splitter into a reference beam and a measuring beam. The measuring beam passes through a glass fiber serving as an object with the reference beam and is merged together again at a further beam splitter and irradiated on a single detector element, whose output is examined with a high-frequency spectral analyzer.